Paroxetine, trans(−)-3-[(1,3-benzodioxol-5-yloxy)methyl]-4-(4-fluorophenyl) piperidine, is a serotonin (5-hydroxy-tryptamine; 5-HT) re-uptake inhibitor, having the formula (I): 
Paroxetine is orally administered, inter alia, for the treatment of depression, social anxiety disorders, obsessive compulsive disorder, panic disorder, generalized anxiety disorder and posttraumatic stress disorder.
U.S. Pat. Nos. 4,902,801 and 5,258,517, and EP 223,334 A1, each of which is incorporated herein in its entirety, disclose preparation of paroxetine by condensation of cinnamate of formula 1 with alkyl amidomalonate of formula 2 to produce imide of formula 3, followed by subsequent transformations, as shown in Scheme I below: The '801 and the '517 patents and EP 223,334 A1 disclose catalyzing the reaction of the cinnamate and the amidomalonate with an alkali metal alkoxide, such as potassium tert-butoxide.
One problem with the synthesis of paroxetine is the defluorination of the intermediates. For example, U.S. Pat. No. 6,326,496, discloses a method for preparing paroxetine to minimize the amount of defluorination.
Defluorination is particularly problematic when the condensation of the cinnamate and the alkyl amidomalonate occurs in the presence of a metal alkoxide as disclosed in the '517 and '801 patents. The reaction can produce the undesired impurity of formula 6, wherein the alkoxy group is substituted for the fluorine substituent. 
When the compound of formula 3 is subsequently reduced to obtain the compound of formula 4, the impurity of formula 6 is also reduced in a similar fashion. The reduction of the impurity leads to the intermediate of formula 4, i.e. 1-methyl-3-hydroxymethyl-4-(4′-fluorophenyl)piperidine (“PMA”), being contaminated with the corresponding impurity of formula 7 (shown below), and a final paroxetine product contaminated with the corresponding alkoxy impurity. 
Thus, at each stage of the process leading to paroxetine, an alkoxy impurity corresponding to the desired intermediate, or to paroxetine, may be present. The alkoxy impurities can not be effectively separated from paroxetine or its intermediates by traditional techniques such as recrystallization. It is believed that the polarity and the structure of the alkoxy impurities is too similar to those of paroxetine and its intermediates to allow for effective separation. Thus, there is a need in the art to prepare paroxetine substantially free of alkoxy impurities.